JPH0777755B2 - Polyethylene film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention not only has excellent heat sealability,
The present invention relates to a polyethylene film having good transparency. More specifically, it not only has excellent heat-sealing properties, but also has good transparency and mechanical properties (for example,
The present invention relates to a polyethylene film having excellent tearability) and good hot tack property.

[Conventional technology]

Conventionally, low-density polyethylene, linear low-density polyethylene, polypropylene, and ethylene-vinyl acetate copolymer have been used as heat seal materials.

Low density polyethylene (LDPE) has been most widely used as a sealant material in extrusion laminates and dry laminates. This is because the seal strength is relatively stronger than other materials, the low-temperature sealability is good, and the cold resistance is excellent. However, it is difficult to stretch, it is difficult to extremely change the vertical-horizontal orientation balance, and it is difficult to obtain a film having good tearability. Further, there is a drawback that the hot tack property, which is important for high speed heat sealability, is poor.

Further, the linear low-density polyethylene and the ethylene-vinyl acetate copolymer have a strong heat-sealing property, are suitable for packaging liquid such as water, and have a good hot tack property. However, when the bag is manufactured at a high speed, the film strength is too strong and the cutting property during bag manufacturing is poor.

Furthermore, regarding polypropylene, it has an appropriate heat seal strength and hot tack property, and since it has a linear molecular structure, it is also excellent in tearability, and also has good transparency, and further, it has excellent transparency during automatic bag making. It has good cutting properties and is therefore widely used in the automatic packaging field. However, at the present time, it is desired to further increase the speed of bag making, and polypropylene films have a limitation in high-speed bag making.

Further, in the field of laminated sealant materials and packaging bags for single-overlap use, especially in the field of food packaging bags, there is a demand for a transparent film that has good heat sealability and allows the contents to be viewed directly. Cast polypropylene (CPP) and LDPE films are widely used as films satisfying these requirements. Of these, the LDPE film is difficult to stretch as described above, and has a problem in tearability and hot tack property. In addition, CPP has a problem in terms of high-speed bag-making property.

[Problems to be solved by the invention]

As described above, in the conventional packaging materials, not only the transparency is excellent, but also the elasticity is strong, and the heat-sealing property and the hot-tack property are good, and a film having easy tearing property is obtained. There wasn't.

From the above, the present invention is a film that does not have these drawbacks (problems), that is, it has excellent transparency, and since it has a high viscosity at the time of heat sealing, it also has good hot tack properties and also has a high heat sealing strength. An object of the present invention is to obtain a film which is excellent, has excellent tearability while maintaining good sealing property, and has sufficient rigidity.

[Means and Actions for Solving Problems]

According to the present invention, these problems are the polyethylene-based film stretched 1.1 to 3.5 times, the polyethylene has a weight average molecular weight (hereinafter referred to as "Mw")
At least an ethylene polymer having a weight average molecular weight (Mw) / number average molecular weight (hereinafter referred to as “Mn”) of 10 or more and a density of 0.935 g / cm ³ or more The polyethylene-based film, which is characterized in that the density of the film is 0.930 g / cm ³ or more and the haze value is less than 5%, can be solved. Hereinafter, the present invention will be specifically described.

(A) Ethylene Polymer The ethylene polymer used in the present invention has an Mw of 150,000 to 300,000, and preferably 150,000 to 250,000. If an ethylene-based polymer having an Mw of less than 150,000 is used,
High viscosity at the time of melting cannot be achieved and hot tackiness is not good. On the other hand, if more than 300,000 ethylene-based polymers are used, it becomes difficult to mold the film using a usual molding machine currently used when manufacturing the film.

Also, the weight average molecular weight / number average molecular weight (that is, Mw / M
n) is 10 or more, preferably 12 or more, especially 12 to 2
5 is preferred. If an ethylene polymer having an Mw / Mn of less than 10 is used, high viscosity cannot be achieved over a wide temperature range, and the film moldability is poor.

Furthermore, the density is 0.935 g / cm ³ or more, 0.935 to 0.975 g /
cm ³ is preferable, and 0.940 to 0.975 g / cm ³ is particularly preferable.
If an ethylene polymer having a density of less than 0.935 g / cm ³ is used, it is difficult to obtain a film that is stretched and has excellent tearability. Low density polyethylene obtained by radical polymerization having a density of less than 0.935 g / cm ³ has many long chain branches,
Not only is it difficult to stretch, but even when stretched, a film with good tearability that does not achieve extreme anisotropy like medium to high density polyethylene with a density of 0.935 g / cm ³ or more is obtained. Can't get

The ethylene polymer used in the present invention is industrially produced by any of the so-called Ziegler method, Phillips method, and standard method, and is used in various fields, and its production method is also widely known. It is what The ethylene-based polymer may be an ethylene homopolymer or a copolymer of ethylene and α-olefin. In this copolymer, the number of carbon atoms of the α-olefin is usually 12 at most, and 3 to 8 is preferable. Representative examples of desirable α-olefins include propylene, butene-1, hexene-1 and 4-methylpen-1. Further, the copolymerization ratio of α-olefin is generally at most 10% by weight,
Particularly, 7% by weight or less is preferable.

(B) Other mixed components When the film of the present invention is produced, only the ethylene polymer may be used, and the ethylene polymer may be a low density polyethylene produced by radical polymerization described below and / or the low density polyethylene. A copolymer with a monomer containing ethylene as a main component, which is produced by a polymerization method, may be further added. Moreover, you may add an additive further.

Both the low-density polyethylene and the copolymer containing ethylene as a main component are obtained by polymerizing ethylene alone or ethylene with another monomer in the presence of a radical catalyst (usually an organic peroxide). Typical examples of other monomers generally have at most 12 carbon atoms.
(Preferably 8 or less) vinyl ester (for example, vinyl acetate), alkyl (meth) acrylate (for example, methyl (meth) acrylate) and unsaturated carboxylic acid or an anhydride thereof (for example, acrylic acid, methacrylic acid) , Maleic acid, maleic anhydride). The copolymerization rate of the monomers is generally at most 15% by weight.

Examples of additives include stabilizers against light, heat and oxygen, which are widely used in the field of ethylene polymers, antistatic agents, fillers, lubricants, plasticizers and the like.

These low-density polyethylene, ethylene-based copolymers and additives are well known in the polyethylene field.

The mixing ratio of the low-density polyethylene and / or the copolymer varies depending on their types and the like, but is generally at most 40% by weight. When adding additives,
Its blending amount is at most 10% by weight.

(C) Mixing method The low density polyethylene is added to the ethylene polymer of the present invention,
Of the ethylene-based copolymers and fillers,
In the case of blending either one, a method generally used in the field of ethylene resins may be applied.

Examples of the mixing method include a dry blending method using a mixer such as a Henschel mixer, and a melt kneading method using a mixer such as a single-screw or twin-screw extruder, a roll mill, a Banbury mixer and a kneader. A method of dry-blending in advance and further melt-kneading the obtained mixture, and a method of previously melt-kneading and further melt-kneading the obtained mixture are preferable because a uniform mixture (composition) can be obtained. Also,
When melt-kneading, it is convenient for forming a film described below to form a pellet.

(D) Film and Method for Producing the Same The film of the present invention is produced by applying a film forming method (for example, an inflation method, a T-die method) generally used in the field of ethylene-based resin to produce a film original fabric in advance. The film of the present invention can be produced by pressurizing a raw material film consisting of two or more having gloss and using a heated roll. The temperature of each roll is lower than the melting point of the ethylene polymer or its composition, but needs to be higher than the melting point of -50 ° C.

Furthermore, the film can be dry laminated with a biaxially oriented polypropylene film, a polyethylene terephthalate film, a nylon film or the like to form a packaging material. At this time, it is desirable that the film of the present invention is surface-modified in advance by a corona discharge treatment or the like.

The film can be used alone as a packaging material such as automatic packaging.

The density of the film of the present invention thus obtained is 0.93.
It is 0 g / cm ³ or more. When the density of the film is less than 0.930 g / cm ^3, tearability is poor.

The stretching ratio of the film is 1.1 to 3.5 times, and particularly 1.1 to 3.0 times is preferable. A film having a draw ratio of less than 1.1 times has poor tearability. On the other hand, if it exceeds 3.5 times, the tear strength is good, but when tearing, a thread-like beard is generated in the tear portion, which is not suitable for packaging foods and the like, and the sealing property is not good.

Further, the thickness of this film is generally 10 to 50 μm, and particularly preferably 15 to 50 μm. When the film has a thickness of less than 10 μm, it is difficult to make it transparent and sufficient heat sealing strength cannot be obtained. On the other hand, if it exceeds 50 μm, it becomes difficult to obtain a film having a haze value of 5% or less.

The film thus obtained has a haze value of 5% or less, so that the contents of the package can be visually observed and has a display effect.

[Examples and Comparative Examples]

Hereinafter, the present invention will be described in more detail with reference to Examples.

In the examples and comparative examples, the heat-sealing strength was obtained by cutting a tanzaque-type test piece (15 mm wide), heat-sealing the sample at a sealing pressure of 2 kg / cm ² for 1 second, and using a tensile tester to measure 300 mm. The strength when peeled at a speed of / minute was evaluated. The hot tack property was determined by firmly adhering the sample to a biaxially oriented polypropylene film (OPP) by a dry lamination method, and used as a sample for hot tack. The sample was sealed at a sealing pressure of 2 kg / cm ² ,
Immediately after heat sealing for 1 second, 25g x 2
The peeling distance at the time of peeling under the load was evaluated. Further, the tear strength was measured according to JIS Z1702. The weight average molecular weight (Mw) was measured at 140 ° C. by gel permeation chromatography using o-dichlorobenzene as a solvent. In addition, haze was measured according to ASTM D1003 for films whose thickness is shown in Table 1.

Examples 1 to 3, Comparative Examples 1 to 5 Density is 0.950 g / cm ³ , Melt Flow Index (measured according to JIS K7210, Condition 4) is 0.04 g / 10 minutes, Mw
Is about 200,000 and high density polyethylene 80 parts by weight with Mw / Mn of 18 and a density of 0.923 g / cm ³ , low density with Mw of about 150,000 and Mw / Mn of 8. A composition was prepared by previously dry blending 20 parts by weight of polyethylene. The composition thus obtained has a density of 0.945 g / cm ³
Met. The composition thus obtained has a temperature of 190.
Inflation molding was carried out under conditions of a temperature of 30 ° C., a blow ratio of 3.0, and a take-up speed of 30 m / min to produce films of 25 μm, 35 μm and 150 μm in thickness.

Of the films thus obtained, the thickness is 35 μm
The film No. 1 was pressed by a film forming machine having two gloss rolls heated to 110 ° C. to produce films stretched at a draw ratio of 1.3 times and 1.5 times (Examples 1 and 2). Further, with respect to the film having a thickness of 150 μm, the film was similarly pressed to draw a film having a draw ratio of 6 times (Comparative Example 2).

The film having a thickness of 25 μm was used without being stretched (Comparative Example 1).

High density polyethylene with a density of 0.960 g / cm ³ , a melt flow index (measured according to JIS K7210 under conditions 4) of 0.8 g / 10 minutes, an Mw of about 100,000 and an Mw / Mn of 5 ( HDPE) was inflation-molded under the same conditions as above to produce films with thicknesses of 25 μm and 35 μm.

Of the films thus obtained, the thickness is 35 μm
For the film No. 1, the stretching ratio is 1.5 by pressing with a molding machine having two gloss rolls heated to 115 ° C.
A double stretched film was produced (Comparative Example 6).

The film having a thickness of 25 μm was used as it was without stretching treatment (Comparative Example 3).

Propylene homopolymer (PP) with melt flow index (measured according to JIS K7210, measured under conditions of 14) of 6.0 g / 10 minutes is equipped with T dyne under conditions of temperature 230 ° C and take-up speed m / min. 25μ thickness using a film forming machine
(Comparative Example 4) Furthermore, the melt flow index (measured according to JIS K7210, condition 4) was 1.0 g / 10 minutes, and the density was 0.9.
23 g / cm Temperature low density polyethylene (LDPE) which is ³ 170
Inflation molding was carried out under conditions of ℃, blow ratio of 1.5 and take-off speed of 15 m / min to produce a film having a thickness of 25 μm (Comparative Example 5).

Each film thus obtained has a melt flow index (measured under conditions of 14 according to JIS K7210).
A propylene homopolymer having a weight of 2.1 g / 10 minutes was molded by a T-die method, and stretch-molded by a sequential biaxial stretching method to prepare films laminated with a film having a thickness of 20 μm by a dry lamination method.

The tear strength, cuttability and haze value of each laminated film were measured. The results are shown in Table 1. Moreover, the hot tack property was measured. The results are shown in Table 2.
In Table 2, the numerical value is the peeling distance. Further, the heat seal strength was measured. The results are shown in Table 3.

[Effect of the Invention] The polyethylene film of the present invention exhibits the following effects.

(1) Heat seal strength is strong.

(2) Excellent hot tack property.

(3) Easy tearability and no stringing phenomenon.

(4) It has a high Young's modulus.

(5) Good transparency.

Since the polyethylene film of the present invention has the above effects, it can be used in various fields. Typical uses are shown below.

(1) Inner bag for confectionery (candy, chocolate, etc.), sealant material for laminating food, industrial material packaging bag (particularly automatic packaging bag) (2) Popsicle bag, Chikuwa packaging bag Packaging bags for single horizontal pillows (3) Single packaging bags for cigarettes, caramel packaging, integrated packaging, etc.

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Claims (1)

[Claims] 1. A polyethylene-based film stretched 1 to 3.5 times, wherein the polyethylene has a weight average molecular weight of 150,000 to 300,000 and a weight average molecular weight / number average molecular weight of 10 or more. In addition, at least an ethylene-based polymer having a density of 0.935 g / cm ³ or more is contained, and the film has a density of 0.930 g / cm ³ or more and a haze value of less than 5%. Polyethylene film.